Cartridges for shotgun-shooting have been the subject of heated discussions among hunters and shooters for the last century. Theories relating to cartridge action vary from time to time, as do opinions on how a cartridge case and the components therein should be constructed.
Historically, the development of cartridge cases has progressed from brass cases, cardboard/metal-base cases, and plastic/metal-base cases to cases manufactured entirely from plastic.
With few exceptions, cases of the latter type have, however, remained on the drawing board.
The specific requirements for shotgun cartridge cases are particularly demanding. A utilisable case must be able to tolerate, for example, a gas pressure of up to 800 Bar. Its diameter should be such that it corresponds to nominal gun calibers of .+-.0.1 to 0.5 mm. After firing, the cartridge case should be easy to remove using an extractor or an ejector. Leakage of powder gas should be minimal. The case should be adaptable to all types of shotguns, including semi-automatic shotguns.
Naturally, the case must not rupture or leave residue in the gun, which could result in barrel rupture with catastrophic consequences.
The base of the cartridge case serves to form a shape-stable bottom with a seat for the primer. The base also forms the abutment plane against the gun's action body.
The case sleeve, particularly its front portion, should create as tight a seal as possible against the bore of the barrel under the influence of the gas pressure from the powder charge and the case sleeve should also be such that it can accommodate the required components, i. a. powder, wad and shot. The front portion of the case sleeve is advantageously such that it can be upset to thereby create a closure and hold the shot charge in place against the wad.
As is noted above, attempts have been made to manufacture shotgun cartridge cases entirely from plastic. It has been recognized that the material costs can thus be reduced and that manufacturing costs can also be lower since it allows for rational manufacturing methods. Furthermore, with regard to environmental and recycling aspects, there are advantages to be gained, particularly if it were possible to manufacture the shotgun cartridge case solely of a plastic material (mono-material).
One method for manufacturing shotgun cartridge cases of plastic which has been used on occasion is based on manufacturing the various parts of the cartridge case of mutually separate plastic materials. Thus, for the base, a stronger, i.e. more pressure-resilient, and more shape-permanent plastic material is selected than that used for the sleeve portion.
One example of this is shown in PCT Application No. WO 86/05871. So-called sequential injection molding is used in the manufacturing of the cartridge in which, in a first step, the sleeve portion is injected and, in connection thereto, in a second step the base portion is injected using a different plastic material than that of the sleeve portion. Furthermore, in this cartridge construction, the sleeve wall in the region of the charge and the primer is made so as to consist of two material layers which together provide a stable base.
Another example of manufacturing the base in a separate step using a different plastic material than that used for the sleeve body is shown in German Application No. 2,419,577. By a special design of the primer hole with three mutually different diameters, the intention here has been to reduce the risk of powder gas release through the primer hole to thereby reduce the risk of displacement of the primer.
An example of a case sleeve made from biaxially oriented plastic material, in an attempt to reduce the material thickness, is described in U.S. Pat. No. 5,121,693. The cartridge case in this example is provided with a traditional base made from metal.
In U.S. Pat. No. 3,760,729 there is disclosed a glass fiber reinforced plastic cartridge case. The case is explicitly designed to have a rigid base. There is a groove extending from the end plane of the base portion of the case.
The reasons for having the groove (the term used in the specification of said patent is "coring") are as follows:
1. to avoid uneven cooling and sinking of the molded case; PA1 2. to avoid long molding cycles required by cooling of the thick base; and PA1 3. to avoid physical weakness at the juncture of the thick base with the thin wall of the case. (See col. 4, lines 1-6).
The plastic cartridge case disclosed in U.S. Pat. No. 3,088,405 is also designed to have a rigid base in the embodiments shown in FIGS. 7,9, and 11, and the base of the cartridge is reinforced by a plurality of ribs. In col. 3, starting on line 49 it is stated: "by said ribs 38, tubular section 34 with counter pocket (probably means "primer pocket") 35 is interconnected with the wall of case 31 whereby a firm, rigid structure is provided having adequate support for the primer . . . ".
A shotgun cartridge case made in one piece from plastic is described in U.S. Pat. No. 3,550,531. According to the disclosure of that patent, the base portion is designed to be able to optimally convert powder gas energy into propulsive energy for the shot charge and to prevent the case from rupturing. The shaping of the base with weakened waist portions between an internal groove and external recesses does, however, imply that there is a large risk of the primer hole expanding such that powder gas is forced out between the primer and the wall of its hole. The primer can be displaced from its position and make it impossible to break the gun after firing.
It is therefore an object of the present invention to provide a shotgun cartridge case made completely of plastic, and shotgun cartridges made therefrom, in which the cartridge case is made in one piece by injection molding of a single plastic material (mono-material), and in which the base portion is shaped so that it meets current specification demands and functions at least as well as a traditional metal base.